"Io is a natural laboratory for volcanoes," said Dr. Duane Bindschadler, Galileo manager of science operations and planning. "By studying Io close up, we'll learn more about how and when volcanoes erupt and why they act the way they do. This may even help us predict the behavior of volcanoes on Earth."

During the flyby, Galileo's science instruments will study the chemistry, heat distribution, gravity and magnetic properties of Io. For scientists, this thrilling encounter promises to yield a bonanza of pictures and information, but for Galileo engineers, the flyby presents a serious challenge with uncertain results. Io's orbit lies in a region of intense radiation from Jupiter's radiation belts, which could affect the performance of spacecraft systems or even knock out various spacecraft instruments. A mere fraction of the dose that Galileo will receive would be fatal to a human.

"We expect that the spacecraft will survive the flyby, although the radiation may cause its computers to reset or may even cause irreversible damage to critical electronic components," said Wayne Sible, Galileo deputy project manager. "There is a possibility, if enough damage is done to the electronics, it won't survive the flyby. Because of this possibility, we planned the Io encounters for the end of the two- year extended mission. After orbiting Jupiter for nearly four years, the spacecraft has more than fulfilled its mission objectives, so it seems reasonable to take a calculated risk for a much closer look at such a scientifically rich target."

Galileo was originally assigned to spend two years studying Jupiter, its moons and its magnetic environment. When that original mission ended in December 1997, it was followed by a two-year extended mission, scheduled to end in January 2000. While spending the past four years near Jupiter, Galileo has been exposed to radiation on an ongoing basis, which has caused some of its instruments to act up.

To prepare for any possible harm caused by radiation during the Io flyby, engineers have designed sophisticated software to help the spacecraft weed out a true crisis from a minor glitch caused by radiation and respond appropriately.

Galileo, the first spacecraft to orbit Jupiter, has revolutionized our knowledge of Jupiter and its moons and has provided thousands of colorful images. Data from Galileo support the premise of a liquid ocean beneath the icy crust of Jupiter's moon Europa, an intriguing prospect since water is a vital ingredient for life. Thanks to information sent by Galileo, scientists know much more about the weather on Jupiter and the composition of its moons. En route to Jupiter, the spacecraft took the first-ever close-up pictures of asteroids, when it photographed Gaspra and Ida, and it returned historic images of the destruction of comet Shoemaker-Levy 9 as its pieces slammed into Jupiter.

If all goes well with the upcoming Io flyby, the spacecraft will make an even more daring approach of Io on Nov. 25 (Pacific time) at an altitude of only 300 kilometers (186 miles).

New Io images taken by the spacecraft are available at the following website: